A driver drives a vehicle on a road in accordance with a predetermined traveling direction. However, in such a case where there are plural lanes each way, there are some cases where a driver drives a vehicle the wrong-way on the road which the driver has been driving even if the driver intended in the first place to make a U-turn to get to an opposing lane. In this respect, a navigation device which detects wrong-way-travel and warns a vehicle occupant of the wrong-way-travel has been proposed (see Patent Document 1 for example). Patent Document 1 describes a navigation device which previously stores information about a wrong-way-travel prohibited road in a database and issues a warning to a driver if a vehicle travels in an opposite direction from a forward direction of the wrong-way-travel prohibited road.
A U-turn detecting device configured to detect a U-turn has also been proposed (see Patent Documents 2, 3 for example). Patent Document 2 describes a U-turn detecting device configured to determine that a vehicle has made a U-turn if a turning angle of the vehicle is within a predetermined range and an average curvature during the turning is less than a threshold value. Patent Document 3 describes a U-turn detecting device configured to detect that a vehicle has made a U-turn if both a turnaround of a traveling direction of the vehicle and a crossing of a lane marking are detected within a predetermined time duration.
A wrong-way-travel detecting device configured to take an image of a forward region by a front camera mounted on a vehicle and to detect a wrong-way-travel based on image processing has also been proposed (see Patent Document 4 for example). Patent Document 4 describes a wrong-way-travel detecting device. The wrong-way-travel detecting device is configured to detect a wrong-way-travel, if a center divider is detected from the image, based on color information of a traffic sign and sizes of traffic signs on the left hand side and the right hand side of a road. If a center divider is not detected from the image, the wrong-way-travel detecting device is configured to compare the number of lanes stored in road map information and the number of lanes detected from the image data, and then detect a wrong-way-travel if the number of lanes at the right side of the lane where the vehicle is traveling is less than the number of lanes at the left side of the lane where the vehicle is traveling.
However, the methods described in Patent Documents 1 and 4 do not detect a wrong-way-travel until the vehicle actually travels in the opposite direction. Thus, there is a problem in that it is difficult to issue a warning to the driver before the turnaround or before a start of the wrong-way-travel. Similarly, in the case where the U-turn is detected as described in Patent Document 2 or 3, as the vehicle had already turned around toward the opposite direction when the U-turn has been detected, it is difficult to issue a warning to the driver before the turnaround or before a start of the wrong-way-travel. Once the vehicle turns around toward the opposite direction, as it can be difficult for the vehicle to turn back toward the forward direction due to the presence of other vehicles traveling in the forward direction, it is considered to be preferable to issue a warning to the driver before the vehicle turns around.    [Patent Document 1] Japanese Patent Publication No. 2007-139531    [Patent Document 2] Japanese Patent Publication No. 2006-030116    [Patent Document 3] Japanese Patent Publication No. 2006-344164    [Patent Document 4] Japanese Patent Publication No. 2007-140883